How to make a curtain
Final result:




Add a Plane, scale it as you need, go to Edit Mode, subdivide it couple of times - W > Subdivide and select vertices as shown below.





Go to Top Ortho view and move selected vertices along choosen axis.





Go back to Front view select top edge and in Object Data tab, under Vertex Groups add new Group (+ sign), and Assign selected vertices.





Go to Object mode and add Cloth physics to the plane in Physics tab. Check Pinning and select Group. Select preset Silk and enable Self Collision (Quality > 2, Distance > 0.6) under Cloth Collision group. 





Create stripe to tie the curtains and make it wider than curtains. I've used simple torus in this example. Go to Physics tab and set it to Collision. Set Outer and Inner values to the lowest (0.001).





Animate scale of the tie. I've putted two keyframes, first frame and 50. Scale by pressing S > Shift+Z. 




That's it. You can add Subsurf to the plane to make it more smoother and add Crease around whole plane with it. Also you will need to modify the tie using proportional editing and make it more stick to the curtains.

You can apply Cloth modifier and reuse them as you want.

Blend file: